Radio frequency communications systems commonly operate in the presence of interfering sources. Unfortunately, interference may degrade or totally impede communications. One particularly troublesome interference source is a microwave oven.
The microwave oven emits interference in a pulsating manner with a repetition rate equal to that of the power line, and at a 50% duty cycle. In the United States, for example, the nominal repetition period is 1/60 of a second or once every 16.7 msec. Microwave ovens typically do not include any filtering of the half-wave rectified operating voltage applied to the microwave magnetron. In other words, the microwave oven emits a noisy, unstable radio frequency pulse that is controlled by the half-wave rectification of the primary power line frequency. The resulting radio frequency interference is at a nominal frequency of 2.45 GHz, and presents a difficulty with communications systems sharing this band. This frequency range has also been allocated for certain communications systems, such as wireless local area networks (WLANs).
A WLAN is a flexible data communication system that may be an extension to, or an alternative for, a wired LAN within a building or campus. A WLAN uses radio technology to transmit and receive data over the air, thereby reducing or minimizing the need for wired connections. Accordingly, a WLAN combines data connectivity with user mobility, and, through simplified configurations, also permits a movable LAN.
Of course, there are other communications systems, typically used in the home, and that may also be affected by the interference from a microwave oven. For example, a cordless telephone, home automation communication systems, or a wireless audio speaker system may be affected by microwave oven generated interference.
The interference problem presented by a microwave oven is addressed, for example, in U.S. Pat. No. 5,574,979 to West and entitled "Periodic Interference Avoidance in a Wireless Radio frequency Communication System". The patent discloses a hierarchical communication scheme including wireless roaming computing and data collection devices. The system monitors the received signal strength and error rates to determine if the interference is periodic in nature so that the system can predict the future interference and transmit only during off periods of the interference.
When a communication time interval is forced to cut off due to an impending presence of interference, the adjusting protocol can determine when it is most efficient to stop the communication, and can continue the rest of the communication time interval structure after the interference abates. Unfortunately, the system for monitoring received signal strength and error rates is relatively complicated and synchronization may be difficult. In addition, both a forward and return channel are needed, thereby complicating the system for certain applications.
Also related to operating a communication system in the presence of interference, U.S. Pat. No. 5,428,669 to McCarthy discloses a cordless telephone set including a base and a handset. One of the units select alternate frequencies in response to a series of unacknowledged communications, until it receives an acknowledgment on a particular channel. If no acknowledgment is received the system reverts back to the originally selected RF channel.
Unfortunately, a microwave oven presents a source of electrical interference that is not readily filtered, and which is capable of causing disruption of communications in a frequency band of the interference. Prior attempts to address the periodic interference caused by a microwave oven have required relatively complicated circuitry, have required two-way communication, and may not have been robust in the presence of the high interfering signals.